1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an optical system for scanning provided in an image forming apparatus. In the optical system for scanning, light such as a laser beam emitted from a light source is transmitted through an optical element such as a converging lens or an acoustooptic modulation element (i.e., an acoustooptic modulator) so as to form an image on a recording medium.
2. Description of the Related Art
An image exposure device, in which an image is scanned by a laser beam so as to form an image on a photographic printing paper, is used in a digital laboratory system. The image exposure device comprises light sources which emit laser beams of R, G, and B. The laser beams emitted from the respective light sources are modulated based on color image data and are irradiated onto a deflector such as a polygon mirror. Subsequently, the laser beams are deflected in a main scanning-direction by the deflector so as to be irradiated onto the photographic printing paper.
The optical system for scanning provided within the image exposure device includes a polygon mirror, an fxcex8 lens, a cylindrical lens, and the like, together with LDs (laser diodes) or SHG modules. The laser beams are irradiated from the LDs onto the polygon mirror rotating at a high speed. The laser beams irradiated onto the polygon mirror are reflected in the main scanning direction by the polygon mirror and are irradiated onto a recording material such as a photographic printing paper for exposure thereof.
One type of the image exposure device comprises light sources for emitting laser beams of R, G, and B so as to form a color image. Such an image exposure device includes the LDs or SHG modules which emit laser beams having wavelengths corresponding to the respective colors of R, G, and B. A collimator lens for converging the laser beam, an acoustooptic modulation element (AOM) for modulating the laser beam based on image data (density data), and the like are provided for each LD or SHG module.
The laser beams emitted from the LDs or SHG modules are converged by the collimator lenses and enter the AOMs. The AOMs diffract second harmonics of the laser beams at an intensity based on the image data.
In the image exposure device, the laser beams diffracted by the respective AOMs are irradiated onto a point on the polygon mirror. In this way, the laser beams corresponding to the respective colors of R, G, and B are collectively deflected in the main scanning direction so as to expose the recording material.
However, in the optical system for scanning, the respective positions of the optical elements need to be precisely adjusted with respect to the optical axes of the laser beams. In such an optical system for scanning, in addition to adjustment of the positional relationship between the LD and the collimator lens, adjustment of the positional relationship between the collimator lens and the AOM is necessary. When the position of the collimator lens is changed, the positional relationship between the LD and the collimator lens needs to be adjusted again. In order to form a color image, it is necessary to adjust the positional relationship between the LD, the collimator lens, and the AOM for each of the laser beams having the colors of R, G, and B, respectively. Further, the positional relationship between the LD, the collimator lens, and the AOM needs to be adjusted so that the laser beam diffracted by the AOM is irradiated onto a point on the polygon mirror. Therefore, assembly of the optical system for scanning has been very complicated.
Further, changes in the relative positions of the optical elements may be caused by temperature changes, impacts, or the like. Adjustment of the positional relationships between the optical elements is also necessary when such changes in the positions of the optical elements arise.
In view of the aforementioned facts, an object of the present invention is to provide an optical system for scanning in which assembly of optical elements is improved and the positional relationships between the optical elements can be easily adjusted at the time of scanning and exposing a recording material by modulating a laser beam by an external modulator such as an acoustooptic modulation element.
In order to achieve the above-mentioned object, a first aspect of the present invention is an optical system for scanning in which a laser beam emitted from a laser light source is modulated by an external modulator, which is provided as one of a plurality of optical elements, in accordance with an image to be recorded on a recording medium and is subsequently deflected in a main scanning direction by main scanning deflection means so as to scan and expose the recording medium, the optical system for scanning comprising; a first base member to which at least two optical elements sequentially disposed along an optical axis of the laser beam are mounted after the positional relationship between.them is adjusted; and a second base member to which another optical element, which is disposed adjacent along the optical axis to the optical elements mounted to the first base member, is mounted after the positional relationship between the another optical element and a single optical element is adjusted, the first base member being the single optical element.
In accordance with the aforementioned first aspect of the present invention, at least two optical elements sequentially disposed along the optical axis of the laser beam are mounted to the first base member. Further, the optical elements which have been mounted to the first base member are mounted to the second base member together with the another optical element which is disposed adjacent, along the optical axis, to the optical elements.
In this way, the positional relationships between more than two optical elements can be adjusted by repeatedly adjusting the positional relationships between two optical elements. Consequently, assembly of optical elements, and adjustment of positional relationships at the time of assembly are considerably facilitated compared with a case in which positional relationships between plural optical elements are adjusted and the optical elements are assembled at the same time.
In other words, in the first aspect of the present invention, the positional relationship between the at least two optical elements sequentially disposed is adjusted, and the at least two optical elements are assembled on the first base member. The assembled optical elements are regarded as a single optical element as a whole, and the positional relationship between the single optical element and another optical element disposed at a position proximal to the single optical element is adjusted. Further, the optical elements whose positional relationship has been adjusted and which have been assembled on the second base member in the above-mentioned manner are regarded as a single optical element as a whole, and the positional relationship between the single optical element and next optical element is adjusted. The another optical element and the next optical element may be each formed by a plurality of optical elements whose positional relationships have been adjusted and which have been assembled on a single base plate.
As a result, adjustment of positional relationships between plural elements is much easier in a case in which the respective elements are grouped into (for example, two) groups of elements and the positional relationship between these two groups is repeatedly adjusted, than in a case in which the positional relationships of each of adjacent pairs of single optical elements are respectively adjusted one-by-one in sequential order.
In the present invention having the above structure, an acoustooptic modulation element can be used as the external modulator.
Further, in the present invention, the laser light source, and a converging lens for converging the laser beam emitted from the laser light source may be assembled, as the optical elements, on the first base member. Furthermore, the laser light source may be formed by a solid state laser and a wavelength conversion element.
In this case, the laser light source and the converging lens are mounted to the first base member. These optical elements mounted to the first base member are regarded as a single optical element and are mounted to the second base member together with the acoustooptic modulation element which is the external modulator.
In this way, assembly of the optical elements forming the optical system from the laser light source to the external modulator is facilitated. Further, the optical elements can be assembled in such a manner that the laser beam modulated in accordance with the image data is regarded as the laser beam exiting from a single optical element.
Furthermore, for example, the laser beams of the respective colors of R, G, and B may be regarded as the laser beams exiting from the respective single optical element. Accordingly, even when a color image is formed, adjustment at the time of assembling the light sources is facilitated.
Moreover, in the present invention, the laser light source, and a converging lens for converging the laser beam emitted from the laser light source may be assembled, as the optical elements, on the first base member. Further, the laser light source may be formed by a solid state laser and a wavelength conversion element.
In this way, even when flexure is generated between the first base member and the second base member, if this flexure falls within a predetermined range, modulation or the like of the laser beam can be carried out without the need to adjust the positional relationships of the optical elements.
Further, in the present invention, it is more preferable that a material such as covar having a linear expansion coefficient xcex1 of xcex1 less than 10xe2x88x925 [Kxe2x88x921] is used as the first base member.
Even when there are changes in ambient temperature, changes in the positional relationships between the optical elements can be prevented by using, at least for the first base member, a material such as covar (FeNeCo) having a low linear expansion coefficient (thermal expansion rate).
Use of a material having a low linear expansion coefficient is not limited to the first base member. It is more preferable that such a material be used for the second base member as well.